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Group II introns, which are widely believed to be the progenitors of the nuclear splicing machinery (the spliceosome) and its substrates, comprise a large ribozyme (catalytic RNA) and the coding sequence of a reverse transcriptase. Group II introns are found in mitochondrial, chloroplast and bacterial genomes and a majority of them behave as retrotransposons.

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Mitochondrion plays a role in: Energy production  Oxidative phosphorilation (OXPHOS) Maintaining the intracellular homeostasis Protecting the rest of the cell from reactive oxygen species (ROS) Apoptosis  important development and disease

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Genome Structure The mitochondrial genome is a circle, 16.6 kb of DNA. A typical bacterial genome is 2-4 Mbp. The two strands are notably different in base composition, leading to one strand being “heavy” (the H strand) and the other light (the L strand). Both strands encode genes, although more are on the H strand. A short region (1121 bp), the D loop (D = “displacement”), is a DNA triple helix: there are 2 overlapping copies of the H strand there. The D loop is also the site where most of replication and transcription is controlled. Genes are tightly packed, with almost no non-coding DNA outside of the D loop. In one case, two genes overlap: they share 43 bp, using different reading frames. Human mitochondrial genes contain no introns, although introns are found in the mitochondria of other groups (plants, for instance).

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Endosymbiont Hypothesis endosymbiont hypothesis: originally proposed in 1883 by Andreas Schimper, but extended by Lynn Margulis in the 1980s. Mitochondrial ribosomal RNA genes and other genes show that the original organism was in the alpha-proteobacterial family (similar to nitrogen-fixing bacteria) Evidence: – mitochondria have their own DNA (circular) – the inner membrane is more similar to prokaryotic membranes than to eukaryotic. By the hypothesis, the inner membrane was the original prokaryotic membrane and the outer membrane was from the primitive eukaryote that swallowed it. – mitochondria make their own ribosomes, which are of the prokaryotic 70S type, not the eukaryotic 80S type. – mitochondria are sensitive to many bacterial inhibitors that don’t affect the rest of the eukaryotic cell, such as streptomycin, chloramphenicol, rifampicin. – mitochondrial protein synthesis starts with N-formyl methionine, as in the bacteria but unlike eukaryotes. Most of the original bacterial genes have migrated into the nucleus. Eukaryotes that lack mitochondria generally have some mitochondrial genes in their nucleus, evidence that their ancestors had mitochondria that were lost during evolution.